Ignition systems for igniting fuel in turbine engines have been in wide use since the 1950s. Although a great variety of systems exists today, their basic architecture has remained fundamentally unchanged over time. The most typical ignition system for turbine engines is a capacitive discharge type. For this type of ignition system, a switch is typically employed to control the discharge of a storage capacitor into an igniter plug located in a combustion chamber of the gas turbine. Historically, a gas-filled device containing a spark gap has functioned as the switch in these ignition systems. Such devices provide a "passive" switch in that they do not require application of an external power supply in order to function properly. Instead, these devices simply employ a single input and output.
Normally, a gas-filled spark gap device comprises a pair of electrodes enclosed in a gas-tight housing together with some type of mildly radioactive emitter, which provides free ions. As energy is pumped into the storage capacitor of the ignition system, the charge on the capacitor causes an increasing electric field across the pair of electrodes in the spark gap. This field begins to ionize the gas within the housing. At some point in the ionization process, the gas begins to conduct current between the electrodes. The ionized gas then becomes a plasma whose electrical resistance drops substantially, thus allowing the current through the spark gap device to increase suddenly and, thereby, create a spark across the gap at the igniter plug. Thus the spark gap responds to reaching a preset voltage by switching from a high to a low impedance, and in that sense it is both a measurement device and a power switch.
For more recent designs, solid state switches have been substituted for the gas-filled spark gap devices. For example, U.S. Pat. No. 5,053,913 to Dolmovich illustrates a series of SCRs that function as an externally-triggered switch connecting the energy storage capacitor to the igniter plug. Another example of a solid-state switch in an ignition system for turbines is shown in U.S. Pat. No. 5,065,073 to Frus. Although these solid-state switches provide certain advantages with respect to the gas-filled spark discharge devices for reasons set forth in the above-identified patents, they require control circuitry, which increases the parts count for implementing an ignition system. Moreover, the control circuitry that is specifically used to externally trigger the SCR devices requires a regulated power supply. Using a nomenclature adopted herein, these type of solid-state switches are referred to as "active" switches.
One of the advantages of ignition systems employing gas-filled spark gap devices is the simplicity of the implementation of ignition systems using such switches. These switches do not require external control circuitry and, therefore, there is no need for additional circuitry. Using a nomenclature adopted herein, these type of switches are referred to as "passive" switches.
Although the gas discharge spark gap devices offer simplicity and reliable performance over a wide range of ambient conditions, they are not as reliable over time as the active switches that utilize solid-state circuitry. For example, the repeated generation of sparks by way of the ionization of the gas in the spark gap device necessarily results in the erosion of the electrodes, which causes the operating characteristics of the device to vary in time. Also, the gas in the spark gap devices deteriorates with time and repeated spark events. The deterioration of the gas adds to the changing discharge characteristics of the device. This gradual wear of the spark gap devices results in variable characteristics that are unpredictable in the near term and ultimately result in a slow and gradual deterioration of performance in the long term. Consequently, the gas discharge spark gap devices must be periodically inspected in order to ensure that the performance of the device has not so deteriorated as to affect the performance of the overall ignition system.